What does IUPAC mean by these rules?

I don't understand IUPAC's rules/recommendations to name the compound.

P-14.4 NUMBERING

When several structural features appear in cyclic and acyclic compounds, low locants are assigned to them in the following decreasing order of seniority:

(…)

(e) saturation/unsaturation:

(i) low locants are given to hydro/dehydro prefixes (…) and ‘ene’ and ‘yne’ endings;

(ii) low locants are given first to multiple bonds as a set and then to double bonds (…);

(f) detachable alphabetized prefixes, all considered together in a series of increasing numerical order;

(…)

Does hydro or dehydro refer to the $\ce{OH}$ group?

The rule first says that low locants are given to ene and yne endings. The next part of the rule says locants are first given to multiple bonds and then to double bonds. Aren't double bonds multiple bonds? Do multiple bonds refer to triple bonds?

• Hydro/dehydro means having added or removed hydrogen, usually a pair. For example: tetrahydrofuran. I think they mean that triple bonds have higher "seniority" than double bonds, but not both of those are lower than double and triple bonds indicated by "ene" or "yne" in the name. – Zhe Oct 18 '16 at 3:19

1 Answer

When there is a choice for numbering, the criteria given in Subsection P-14.4 are applied in order until a decision can be made.
For example, since Criterion (e) takes precedence over Criterion (f), the systematic name of allyl chloride is 3-chloroprop-1-ene, not 1-chloroprop-2-ene, because low locants are given first to the double bond (the ‘ene’ ending) and then to the ‘chloro’ prefix.

The term ‘multiple bonds’ includes double bonds and triple bonds. According to Criterion (e) (ii), low locants are given first to multiple bonds as a set, even though this may at times give triple bonds (‘yne’ endings) lower locants than double bonds (‘ene’ endings).
For example: a correct systematic name is pent-3-en-1-yne, not pent-2-en-4-yne, because the locant set 1,3 is lower than 2,4.
Only if a choice remains, preference for low locants is given to the double bonds in accordance with Criterion (e) (ii) (“… and then to double bonds …”).
For example: a correct systematic name is pent-1-en-4-yne, not pent-4-en-1-yne, because the locant 1 for the double bond (the ‘ene’ ending) is lower than 4.
(See also Subsection P-31.1.1.1.)

The prefixes ‘hydro’ and ‘dehydro’ are not related to hydroxy groupds ($\ce{-OH}$). They are used to indicate addition and subtraction, respectively, of hydrogen atoms (see Subsection P-31.2.1).
For example: a systematic name for benzyne is 1,2-didehydrobenzene, which indicates the removal of two hydrogen atoms.